Enabling GRE Support

If you're trying to connect a VPN client to a Microsoft PPTP connection and you are using a Linux box as a firewall for your broadband connection, you'll have to take an additional step. Allow Generic Route Encapsulation (GRE) protocol to pass through the firewall. If, for example, you are using iptables on your Linux firewall and your VPN server has the IP address 189.44.45.3, you would enter the following:

iptables -I FORWARD -p 47 -d 189.44.45.3 -j ACCEPT

Routing Packets through the Connection

Sometimes you'll find it necessary to explicitly route packets through a specific interface. Many Windows administrators consider this one of the biggest challenges in working with Linux clients.

The need for explicit routing is especially important when your remote network is using public IP addresses. Even though you have a VPN tunnel, your network interface might still try to route packets across the Internet, rather than through the VPN tunnel. Many times when I have helped troubleshoot "failed" VPN connections, all I had to do was simply add a few alternative routes to the default routing table. Sometimes I would do this with the route command (as root). Other times I used the VPN GUI application.

The route command is the standard:

route add -net 13.163.97.23 netmask 255.255.255.255 dev ppp0

Also, you can use the ip command

ip route add 171.87.44.54/24 dev ppp0

or specify the routes with the use of the VPN software GUI interface. In some cases, if you don't add these routes, the packets that you intended to go through the VPN tunnel will be routed through your wireless or Ethernet card instead of your VPN interface.

GUI Options

The GUI VPN applications are getting much better at adding routes on their own. Figure 1 shows the settings for KVpnc. Figure 2 provides a similar configuration in NetworkManager.

Figure 1: Adding routes in a KVpnc VPN client.

Figure 2: Adding routes in the NetworkManager client.

As these images show, packets that match the IP address and subnet mask will not be sent across a standard network connection; rather, they will be sent through the VPN tunnel.

KVpnc (Figure 3), which is supported by many distributions, is perhaps the most versatile client in that it supports L2TP, Cisco free and proprietary VPN protocols, and OpenVPN and Microsoft PPTP. KVpnc also lets you import certificates, as shown in Figure 4.

Figure 3: KVpnc running a legacy PPTP session.

Figure 4: Importing a Cisco certificate in KVpnc.

Even though the user interface programmers can't spell particularly well (notice the word "proprietary" is misspelled in my version of the program; Figure 4), the KVpnc team has created an implementation that works particularly well with Cisco devices.

The venerable pptpconfig tool also is available with many distributions. The key to getting pptpconfig to work properly is to make sure the encryption settings are configured exactly as your network administrator has them set.

I've often found that requiring MPPE encryption and enabling stateful MPPE encryption are important. In the case of pptpconfig, you would select "Require Microsoft Point-to-Point Encryption (MPPE)" and "Refuse Stateless Encryption" to accomplish this.

Pptpconfig also has the ability to add routes automatically. Simply click the Routing tab, then select the Client to LAN radio button and enter the routes of the systems you want to connect with through your VPN tunnel.

Many Linux users prefer the NetworkManager client for one simple reason: It tends to work. Plugins are available for NetworkManager that support various protocols, including OpenVPN, Microsoft PPTP, and Cisco's L2TP methods. In my Ubuntu system, I use apt-get, but you can also search for the appropriate plugins with Synaptic. Once you've added the plugin, all you have to do is click on the network icon then select VPN connections to begin entering the appropriate information. Figure 5 shows the steps for configuring a Cisco connection on an Ubuntu system.

Figure 5: Configuring a Cisco VPN connection in NetworkManager.

NetworkManager supports both shared-key and X.509 certificate-based encryption. The keys to getting NetworkManager to function properly include installing the racoon daemon software to handle the Internet Key Exchange, creating or otherwise obtaining a shared key from your administrator, and creating or otherwise obtaining signed certificates from your administrator (if you are using certificates). Also, you have the option of importing the saved configuration files of existing connections.